The immune response to polysaccharide (PS) antigens is highly regulated and has several distinguishing features including restricted subclass, variable region gene usage, fine specificity, and avidity. The response is often different depending on the form that the PS antigen is presented to the immune system. Simple PS not conjugated to protein (such as bacterial levan (BL) or Neisseria meningitidis group C (MCPS)) elicit a thymus-independent (TI) response. Our earlier studies of the TI-response to BL in two strains of mice (BALB/c and CBA/Ca) showed strain-specific differences in monoclonal antibody (mAb) VH and VL gene family usage and fine specificity. BALB/c mAb exhibited a highly restricted response, being primarily VHJ606/VK11 and cross-reacted with inulin (In). CBA/Ca mAb, on the other hand, used VHJ606, VK11 and could cross-react with In but primarily did not. DNA sequence and genomic Southern data of VHJ606 and VK11 genes indicate that BALB/c anti-BL mAb use essentially the same germline VH-DH-JH gene combination whereas CBA/Ca anti-BL mAb use a different germline VH gene in combination with a variety of DH and JH gene segments with VH-DH-JH junctional diversity and significant somatic mutation. VK11 gene usage by the strains appears identical. Ongoing studies to investigate regulatory mechanisms involved in modulating this strain-specific TI response include germline gene repertoire analysis and measurement of mAb affinity relative to fine specificity in both strains. PS conjugated to proteins (such as MCPS coupled to tetanus toxoid (MCPS- TT)), on the other hand, elicit a different type of response termed thymus-dependent (TD). Previous studies indicated that immunization with MCPS-TT compared to MCPS resulted in isotype change, increased diversity and increased avidity in response to the TD form. Dot blot and Northern analyses of anti-MCPS mAb reveal that VH gene family usage is dominated by VHJ558 (the largest VH gene family) and appears to correlate with VH gene family size in a random fashion in response to either the TD or TI form. Fine specificity and VH family usage do not seem to correlate with the exception of VH3609 mAb which are specific for MCPS and are only found in response to the TI form. Sequence analysis in progress will determine if somatic mutation can account for the increased diversity and increased avidity of the immune response to MCPS-TT over MCPS alone. An understanding of antibody diversity patterns and immunoglobulin gene structure and usage in response to both TD and TI antigens is essential for developing expertise in genetic engineering of mAbs.